1. Field of the Invention
Aspects of the present invention relate to an organic electroluminescence display and a driving method thereof. More specifically, aspects of the present invention relate to an organic electroluminescence display capable of displaying a gray level using a frequency characteristic of an organic electroluminescence device, and a driving method thereof.
2. Description of the Related Art
Flat panel displays contain a plurality of pixels in a matrix arrangement on a substrate and have the pixels set as a display area. In the flat panel displays, scan lines and data lines are connected to pixels to display an image by selectively applying data signals to the pixels.
The flat panel displays are classified into different type displays according to a driving mode of a pixel, including a passive matrix-type light-emitting display and an active matrix-type light-emitting display. The active matrix-type light-emitting display which emits light from every pixel has been used mainly due to better resolution, contrast, and operating speed.
The active matrix-type light-emitting displays are used as displays for such devices as a personal computer, a portable phone, PDA, etc., or as monitors of various information appliances even though various other types of flat panel displays are known in the art. Other types of flat panel displays include liquid crystal displays (LCDs) using a liquid crystal panel, organic electroluminescence displays using an organic electroluminescence device, and plasma display panels (PDPs) using a plasma panel, etc.
Recently, various light-emitting displays have been developed having a smaller weight and volume than a cathode ray tube, and attention has been particularly paid to organic electroluminescence displays which are excellent in luminous efficiency, luminance and viewing angles, and have rapid response times.
FIG. 1 is a view of a circuit showing a pixel used in one related art organic electroluminescence display. Referring to FIG. 1, the pixel is formed on a region where a data line (Dm) and a scan line (Sn) are crossed, and includes a first transistor (T11), a second transistor (T21), a capacitor (Cst), a compensation circuit 11, and an organic electroluminescence device (OLED). During operation, the pixel is selected by receiving a scan signal through the scan line (Sn), and a data signal is transmitted to the selected pixel through the data line (Dm) so that a luminance corresponding to the data signal is displayed. Also, each pixel is operated by receiving power from a first power supply (ELVdd) and a second power supply (ELVss).
The first transistor (T11) allows a current to flow from a source to a drain according to a signal applied to a gate electrode, and has a gate connected to the compensation circuit 11, a source connected to the first power supply (ELVdd), and a drain connected to the organic electroluminescence device (OLED).
The second transistor (T21) transmits a data signal to the compensation circuit 11 according to the scan signal, and has a gate connected to the scan line (Sn), a source connected to the data line (Dm), and a drain connected to the compensation circuit 11.
The capacitor (Cst) applies a voltage to the compensation circuit 11 that corresponds to the data signal. The capacitor (Cst) maintains a voltage of the data signal during a predetermined period. Therefore, the first transistor (T11) allows a current that corresponds to the voltage of the data signal to flow during a predetermined period. As a result, even if the data signal is interrupted by the second transistor (T21), since the first electrode is connected to the first power supply (ELVdd) and the second electrode is connected to the compensation circuit 11, the second electrode maintains a voltage that corresponds to the data signal. Accordingly, the voltage that corresponds to the data signal is maintained on the gate of the first transistor (T11) during the predetermined period.
The compensation circuit 11 compensates for a threshold voltage of the first transistor (T11) by receiving a compensation control signal. Accordingly, the compensation circuit 11 prevents unevenness of a luminance due to unevenness of a threshold voltage. The compensation control signal may be transmitted by an additional signal line or may be transmitted by the scan line.
The organic electroluminescence device (OLED) has an organic film formed between an anode electrode and a cathode electrode so that the organic film is allowed to emit light. Light is emitted from the organic film if a current flows from the anode electrode to the cathode electrode. In the OLED shown in FIG. 1, the anode electrode is connected to the drain of the first transistor (T11) and the cathode electrode is connected to the second power supply (ELVss). The organic film includes an emitting layer (EML), an electron transport layer (ETL) and a hole transport layer (HTL). Also, the organic electroluminescence device may further include an electron injection layer (EIL) and a hole injection layer (HIL).
FIG. 2 is a view of a circuit showing another pixel used in a related art organic electroluminescence display. Referring to FIG. 2, the pixel includes a first transistor (T12), a second transistor (T22), a third transistor (T32), a fourth transistor (T42), a capacitor (Cst), and an organic electroluminescence device (OLED). The OLED shown is referred to as a current-driving pixel circuit for controlling a luminance using a current.
During operation of the current-driving pixel circuit, when the second transistor (T22) and the third transistor (T32) are in an ON state based on the scan signal, a current is generated in the first transistor (T12) that corresponds to a current flowing to the data line. At this time, a voltage corresponding to a capacity of the current is stored in the capacitor (Cst). Thereafter, when the second transistor (T22) and the third transistor (T32) are in an OFF state, the first transistor (T12) allows a current to flow to the organic electroluminescence device (OLED) due to the voltage stored in the capacitor (Cst). The current-driving pixel circuit as configured above does not have problems arising from an unevenness of a threshold voltage, etc., since the circuit uses the flowing current.
As described above, the pixel as shown in FIG. 1 should include a circuit for compensating for an uneven threshold voltage, while the pixel as shown in FIG. 2 is not suitable for a large screen of the organic electroluminescence display since time needed for charging by a current is increased due to a parasitic capacitor, etc., and since the driving circuit is more complicated.